As is known in the prior art, it is desirable to cyclically modulate the gain of the receiver of a pulsed radar system. In such a system the power associated with the radio frequency echoes from a target varies inversely with the fourth power of the range (inversely with the fourth power of the two-way propagation time of the radar energy). Commonly such radar systems employ sensitivity-time control (STC) in the receiver thereof. Without an STC, a mixer, IF or RF amplifier of the receiver would require an extremely wide dynamic range for processing signals having a large variation in signal strength as an inverse function of range. The STC reduces the dynamic range requirement of the mixer or amplifier by varying the gain (i.e. sensitivity) of the receiver, ideally as the fourth power of the propagation time of the radar energy, that is, in inverse relationship to the reduction in power associated with received echo signals from increasing ranges.
Prior art STC's may use an analog attenuator, typically comprised of pin diodes synchronized with each one of the transmitted pulses. The gain of such attenuator increases in accordance with the fourth power of the time interval after each one of such transmitted pulses. Some disadvantages such as signal distortion, drift, and poor reliability are associated with analog attenuators.
While a digitally controlled attenuator does not have the inherent disadvantages of the analog attenuator, prior art digital attenuators using field effect transistors may not be able to operate satisfactorily with radio frequency signals (that is of frequencies in a typical 2-4 GHZ range). Because of their relatively high switching speed, radio frequency signals can couple through an "off" FET because of its inherent interelectrode capacitance, and thereby prevent accurate control of the desired attenuation factor for the attenuator. Further, where a ladder network is used in the digital attenuator and portions of the radio frequency signals are coupled into selected shunt elements of the ladder network, the relative phase shift between signals passing through different selected shunt elements may have significant effect on accurately establishing a desired attenuation factor for the digital attenuator.
U.S. Pat. No. 3,765,020 discloses a technique for overcoming the aforementioned disadvantages of a digitally controlled attenuator. In particular, means are provided for coupling a first portion of the radio frequency signals to a compensator and a second portion thereof to a radio frequency bus. The radio frequency bus has connected thereto, at predetermined points, a plurality of switching and coupling networks, each one thereof used for coupling, in proper phase relationship, a part of the second portion of the radio frequency signals to a shunt element of a ladder network in accordance with a digital control signal. The output of the ladder network and the output of the compensator are combined in a manner such that any unwanted signals passing through the switching networks are effectively cancelled.
Although ths patent purports to correct the deficiencies attendant with the use of digital attenuators, use of such digital attenuators still have serious disadvantages. In particular, high levels of switching noise are present in the system and compensation means are required for the phase shift effects.
U.S. Pat. No. 4,106,872 discloses a technique for measuring cloud altitude in which the sensitivity of a receiver for detecting echo signals from the clouds is controlled to adjust the receiver for detection of only clouds, not haze, for example. A repetitive waveform is read out of a ROM with range and the output is converted to generate a specified waveform which in turn is utilized to adjust the sensitivity of a level sensing unit. A technique for adjusting the ROM output waveform to meet various operating requirements is not disclosed however.
Additional disadvantages of prior art STC circuits are as follows:
(1) Piecewise approximations to R.sup.-4, R.sup.-3, etc., range attenuation profiles are utilized and are inherently inaccurate at points across the dynamic range. PA1 (2) STC is provided by control of FET gate voltage, transistor emitter current, or vacuum tube grid voltage, using several stages to achieve the full STC dynamic range. PA1 (3) The control parameter is linear over some dynamic range, with "knee points" which vary from device to device, thus unduly complicating matching to the piecewise approximation of R.sup.-4, etc., curves. PA1 (4) The piecewise-approximate STC curves are extremely unwieldy for slope changeover (e.g. R.sup.-4 to R.sup.-3) on a dynamic basis. PA1 (5) The piecewise-approximate STC curves are also unwieldy for STC range end-point changeover on a dynamic basis. This is a substantial impediment with power-programmed radars where STC range end-point needs to change dynamically, group-to-group.
The prior art digitally programmed electronic attenuators set forth hereinabove have additional disadvantages including excess insertion loss. Due to the nature of the circuitry, the residual insertion loss tends to increase on a per-bit basis. A 6-bit digital attenuator at S-band has a specification of 0.6 to 0.7 db per bit, or 3.6 to 4.2 dB residual insertion loss. Further, these attenuators generally reconvert to analog voltage for control of a single analog attenuating circuit.
What is desired is to provide simplified apparatus for modifying the STC end point or STC slope without requiring access to large memory capacity devices, the added memory greatly increasing STC cost.